1. Field of the Invention
The present invention relates to a drive of an image blur suppression (also known as an image shake correcting means) in a camera which can suppress blurring in an image projected on an image plane (also referred to as correcting image shakes) caused by camera shakes.
2. Related Background Art
The majority of recent cameras have the program AE mode for properly determining the aperture priority AE mode, the shutter priority AE mode, or both by a circuit inside the camera in addition to the manual mode in which a photographer determines the lens aperture and the shutter speed.
There are cameras of a type in which a strobe light source is built in a camera and the strobe photographic mode is automatically set according to luminance of a subject, and of a type in which in strobe photography a photographer can select either a leading curtain synchro mode in which the strobe is flashed in synchronism with the opening of a shutter leading curtain or a trailing curtain synchro mode in which the strobe is flashed immediately before the closure of a shutter trailing curtain.
Among such various exposure methods, a photographic mode is properly selected in accordance with the photographic purpose of photographer, which in turn makes it possible that the photographic purpose of a photographer can be judged by a camera upon selection of a photographic mode.
There is such a technology already disclosed that a camera having some selectable photographic modes as described above is provided with image shake correcting means for correcting image shakes caused by camera shakes and that the drive pattern of image shake correcting means is changed according to the mode selection out of the photographic modes.
For example, Japanese Laid-open Patent Application No. 2-58037 shows an example in which the drive of image shake correcting means is not carried out if the strobe photographic mode is selected.
In considering problems in drive of image shake correcting means during photography, it is necessary to consider a case in which a photographer is about to conduct panning.
Namely, when a photographer moves the camera to conduct panning, the camera with image shake correcting means normally judges that the camera movement is undesirable camera shake. There is thus a great risk of losing the dynamic effect of panning, because the correction drive for suppressing image shakes is effected on the rotation of camera in panning as well. A countermeasure to such a case is also described in above Japanese Laid-open Patent Application No. 2-58037, which shows an example to stop the drive of image shake correcting means.
Either of the above conventional examples describes the countermeasure of conducting no drive of image shake correcting means.
A present trend in exposure methods in strobe photography is to effect strobe flashing in slow shutter (as will be referred to as slow synchro) in which an atmosphere upon photography can be more faithfully depicted while efficiently using the representation of background behind a main subject. Therefore, if the drive of image shake correcting means is always stopped during strobe flashing, the representation of background is not only degraded by camera shakes, but the main subject also forms a blurred image because of stationary light, thus degrading a photographic result.
The unconditional drive stop is more problematic, especially in the trailing curtain synchro mode which is often used to emphasize movement, because the photographic condition of low shutter speed is frequent. Also, in case of the latter panning photography, the image shake correction drive would better be effected on image shakes caused by fine camera shakes other than the smooth camera rotation in the direction of panning to prevent the image of the subject from being unnecessarily degraded. There is, therefore, a problem in this respect in the arrangement that the image shake correction drive is always stopped during panning photography as in the conventional technology.
Now described as a conventional example of a circuit having a time constant is one used in an image-shake preventing camera with a camera shake correcting circuit.
A camera having a conventional camera-shake correcting circuit, as disclosed in Japanese Laid-open Patent Application No. 2-126251, obtains a camera shake angular velocity signal from an angular velocity detecting sensor, and calculates a displacement amount of camera shake through a filter circuit, a drift compensation circuit, and an integration circuit. In the method as described in this publication, no specific countermeasure is described as to the waiting time after switching-on and before a stable state of a circuit, which is normally problematic in filter circuits. The publication shows such a measure only for the integration constant of an integration circuit, which is another important element, that "when the shutter speed is high (that is, when the subject is bright) the integration constant is decreased to allow an instant response to the release, whereas when the shutter speed is slow, that is, when the subject is dark, a warning is given to a photographer to delay the release and the integration constant is increased to allow detection of slow camera shakes."
To obtain a camera shake displacement amount as described above, a signal from the angular velocity detecting sensor is subjected to filtering and the filtered signal is integrated. The circuits having the time constant, such as the filter circuit and the integration circuit used in the above arrangement, include a lot of problems to be considered, as listed below. It cannot be said that the conventional technology as described above fully discloses solutions to these problems.
(1) To obtain a camera shake displacement amount, a filter circuit is necessary for removing drift components and noise components in a signal from the angular velocity detecting sensor in addition to the integration circuit for integrating the signal from the angular velocity detecting sensor. Since a frequency band of camera shake signal obtained from the angular velocity detecting sensor is about 1 to 10 Hz, the cut-off frequency of the filter circuit will be a low frequency of not more than 1 Hz. Such a filter circuit requires a substantial time (several tens of seconds) before the circuit is stabilized after switched on.
(2) The above is also the case as to the integration circuit. Thus, if a subject is dark, a considerably long time is necessary for determining the integration constant.
(3) As described, a considerable time is necessary before the camera shake correcting circuit becomes ready to start a stable operation. A specific measure to solve this problem is necessary. Also, some means is necessary for giving a warning to a photographer before the circuit is stabilized. The above conventional examples disclose no specific warning means.
(4) As for the warning means, a further consideration should be taken. For example, if the warning means is temporary warning means using a warning sound or the like, a photographer could fail to hear the warning sound. There would be a situation that he cannot judge whether he may start shooting even after the filter circuit is stabilized and the integration constant of integration circuit is set. Accordingly, a measure to avoid such a situation is also necessary.